fix: correctly resolve fields of aliased structs

[secDre4mer]

Fields of types that are aliases are not always resolved correctly, especially if the aliased type is itself an alias.

A minimal example is:

package main

type original struct {
	Field string
}

func main() {
	type alias original
	type alias2 alias
	var a = &alias2{
		Field: "test",
	}
	println(a.Field)
}

There is already a method (baseType) for resolving the underlying struct type; but it isn't always used previous to this change.

[mvertes]

Looks good to me